Surprisingly simple explanation for the alien comet 'Oumuamua's weird
orbit
2017 comet's unusual acceleration explained by hydrogen outgassing from
ice

Date:
March 22, 2023
Source:
University of California - Berkeley
Summary:
When the first interstellar comet ever seen in our solar system
was discovered in 2017, one characteristic -- an unexplained
acceleration away from the sun -- sparked wild speculation,
including that it was an alien spacecraft. An astrochemist
found a simpler explanation and tested it with an astronomer:
in interstellar space, cosmic rays converted water to hydrogen in
the comet's outer layers. Nearing the sun, outgassed hydrogen gave
the tiny comet a kick.


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FULL STORY ==========================================================================
In 2017, a mysterious comet dubbed 'Oumuamua fired the imaginations of scientists and the public alike. It was the first known visitor from
outside our solar system, it had no bright coma or dust tail, like most
comets, and a peculiar shape -- something between a cigar and a pancake --
and its small size more befitted an asteroid than a comet.


==========================================================================
But the fact that it was accelerating away from the sun in a way that astronomers could not explain perplexed scientists, leading some to
suggest that it was an alien spaceship.

Now, a University of California, Berkeley, astrochemist and a Cornell University astronomer argue that the comet's mysterious deviations from
a hyperbolic path around the sun can be explained by a simple physical mechanism likely common among many icy comets: outgassing of hydrogen
as the comet warmed up in the sunlight.

What made 'Oumuamua different from every other well-studied comet in
our solar system was its size: It was so small that its gravitational deflection around the sun was slightly altered by the tiny push created
when hydrogen gas spurted out of the ice.

Most comets are essentially dirty snowballs that periodically approach the
sun from the outer reaches of our solar system. When warmed by sunlight,
a comet ejects water and other molecules, producing a bright halo or
coma around it and often tails of gas and dust. The ejected gases act
like the thrusters on a spacecraft to give the comet a tiny kick that
alters its trajectory slightly from the elliptical orbits typical of
other solar system objects, such as asteroids and planets.

When discovered, 'Oumuamua had no coma or tail and was too small and
too far from the sun to capture enough energy to eject much water, which
led astronomers to speculate wildly about its composition and what was
pushing it outward. Was it a hydrogen iceberg outgassing H2? A large,
fluffy snowflake pushed by light pressure from the sun? A light sail
created by an alien civilization? A spaceship under its own power?
Jennifer Bergner, a UC Berkeley assistant professor of chemistry who
studies the chemical reactions that occur on icy rocks in the cold vacuum
of space, thought there might be a simpler explanation. She broached
the subject with a colleague, Darryl Seligman, now an National Science Foundation postdoctoral fellow at Cornell University, and they decided
to work together to test it.

"A comet traveling through the interstellar medium basically is getting
cooked by cosmic radiation, forming hydrogen as a result. Our thought
was: If this was happening, could you actually trap it in the body,
so that when it entered the solar system and it was warmed up, it would
outgas that hydrogen?" Bergner said. "Could that quantitatively produce
the force that you need to explain the non-gravitational acceleration?" Surprisingly, she found that experimental research published in the 1970s,
'80s and '90s demonstrated that when ice is hit by high-energy particles
akin to cosmic rays, molecular hydrogen (H2) is abundantly produced and
trapped within the ice. In fact, cosmic rays can penetrate tens of meters
into ice, converting a quarter or more of the water to hydrogen gas.

"For a comet several kilometers across, the outgassing would be
from a really thin shell relative to the bulk of the object, so both compositionally and in terms of any acceleration, you wouldn't necessarily expect that to be a detectable effect," she said. "But because 'Oumuamua
was so small, we think that it actually produced sufficient force to power
this acceleration." The comet, which was slightly reddish, is thought
to have been roughly 115 by 111 by 19 meters in size. While the relative dimensions were fairly certain, however, astronomers couldn't be sure
of the actual size because it was too small and distant for telescopes
to resolve. The size had to be estimated from the comet's brightness and
how the brightness changed as the comet tumbled. To date, all the comets observed in our solar system -- the short-period comets originating in the Kuiper belt and the long-period comets from the more distant Oort cloud
have ranged from around 1 kilometer to hundreds of kilometers across.

"What's beautiful about Jenny's idea is that it's exactly what should
happen to interstellar comets," Seligman said. "We had all these stupid
ideas, like hydrogen icebergs and other crazy things, and it's just the
most generic explanation." Bergner and Seligman will publish their
conclusions this week in the journal Nature. Both were postdoctoral
fellows at the University of Chicago when they began collaborating on
the paper.

Messenger from afar Comets are icy rocks left over from the formation of
the solar system 4.5 billion years ago, so they can tell astronomers about
the conditions that existed when our solar system formed. Interstellar
comets can also give hints to the conditions around other stars surrounded
by planet-forming disks.

"Comets preserve a snapshot of what the solar system looked like when
it was in the stage of evolution that protoplanetary disks are now,"
Bergner said.

"Studying them is a way to look back at what our solar system used to
look like in the early formation stage." Faraway planetary systems
also seem to have comets, and many are likely to be ejected because
of gravitational interactions with other objects in the system, which astronomers know happened over the history of our solar system. Some of
these rogue comets should occasionally enter our solar system, providing
an opportunity to learn about planet formation in other systems.

"The comets and asteroids in the solar system have arguably taught us more about planet formation than what we've learned from the actual planets
in the solar system," Seligman said. "I think that the interstellar
comets could arguably tell us more about extrasolar planets than the
extrasolar planets we are trying to get measurements of today." In the
past, astronomers published numerous papers about what we can learn from
the failure to observe any interstellar comets in our solar system.

Then, 'Oumuamua came along.

On Oct. 19, 2017, on the island of Maui, astronomers using the Pan-STARRS1 telescope, which is operated by the Institute for Astronomy at the
University of Hawaii in Manoa, first noticed what they thought was either
a comet or an asteroid. Once they realized that its tilted orbit and high
speed -- 87 kilometers per second -- implied that it came from outside
our solar system, they gave it the name 1I/'Oumuamua (oh MOO-uh MOO-uh),
which is Hawai'ian for "a messenger from afar arriving first." It was
the first interstellar object aside from dust grains ever seen in our
solar system. A second, 2I/Borisov, was discovered in 2019, though it
looked and behaved more like a typical comet.

As more and more telescopes focused on 'Oumuamua, the astronomers were
able to chart its orbit and determine that it had already looped around
the sun and was headed out of the solar system.

Because 'Oumuamua's brightness changed periodically by a factor of 12
and varied asymmetrically, it was assumed to be highly elongated and
tumbling end over end. Astronomers also noticed a slight acceleration
away from the sun larger than seen for asteroids and more characteristic
of comets. When comets approach the sun, the water and gases ejected
from the surface create a glowing, gaseous coma and release dust in the process. Typically, dust left in the comet's wake becomes visible as
one tail, while vapor and dust pushed by light pressure from solar rays produces a second tail pointing away from the sun, plus a little inertial
push outward. Other compounds, such as entrapped organic materials and
carbon monoxide, also can be released.

Why was it accelerating? But astronomers could detect no coma, outgassed molecules or dust around 'Oumuamua. In addition, calculations showed that
the solar energy hitting the comet would be insufficient to sublimate
water or organic compounds from its surface to give it the observed non-gravitational kick. Only hypervolatile gases such as H2, N2 or carbon monoxide (CO) could provide enough acceleration to match observations,
given the incoming solar energy.

"We had never seen a comet in the solar system that didn't have a
dust coma.

So, the non-gravitational acceleration really was weird," Seligman said.

This led to much speculation about what volatile molecules could be in
the comet to cause the acceleration. Seligman himself published a paper
arguing that if the comet was composed of solid hydrogen -- a hydrogen
iceberg -- it would outgas enough hydrogen in the heat of the sun to
explain the strange acceleration. Under the right conditions, a comet
composed of solid nitrogen or solid carbon monoxide would also outgas
with enough force to affect the comet's orbit.

But astronomers had to stretch to explain what conditions could lead
to the formation of solid bodies of hydrogen or nitrogen, which have
never been observed before. And how could a solid H2 body survive for
perhaps 100 million years in interstellar space? Bergner thought that outgassing of hydrogen entrapped in ice might be sufficient to accelerate 'Oumuamua. As both an experimentalist and a theoretician, she studies
the interaction of very cold ice -- chilled to 5 or 10 degrees Kelvin,
the temperature of the interstellar medium (ISM) -- with the kinds of
energetic particles and radiation found in the ISM.

In searching through past publications, she found many experiments demonstrating that high-energy electrons, protons and heavier atoms could convert water ice into molecular hydrogen, and that the fluffy, snowball structure of a comet could entrap the gas in bubbles within the ice.

Experiments showed that when warmed, as by the heat of the sun, the
ice anneals -- changes from an amorphous to a crystal structure -- and
forces the bubbles out, releasing the hydrogen gas. Ice at the surface
of a comet, Bergner and Seligman calculated, could emit enough gas,
either in a collimated beam or fan- shaped spray, to affect the orbit
of a small comet like 'Oumuamua.

"The main takeaway is that 'Oumuamua is consistent with being a
standard interstellar comet that just experienced heavy processing,"
Bergner said. "The models we ran are consistent with what we see in the
solar system from comets and asteroids. So, you could essentially start
with something that looks like a comet and have this scenario work."
The idea also explains the lack of a dust coma.

"Even if there was dust in the ice matrix, you're not sublimating the ice, you're just rearranging the ice and then letting H2 get released. So,
the dust isn't even going to come out," Seligman said.

'Dark' comets Seligman said that their conclusion about the source of 'Oumuamua's acceleration should close the book on the comet. Since 2017,
he, Bergner and their colleagues have identified six other small comets
with no observable coma, but with small non-gravitational accelerations, suggesting that such "dark" comets are common. While H2 is not likely responsible for the accelerations of dark comets, Bergner noted, together
with 'Oumuamua they reveal that there is much to be learned about the
nature of small bodies in the solar system.

One of these dark comets, 1998 KY26, is the next target for Japan's
Hayabusa2 mission, which recently collected samples from the asteroid
Ryugu. The 1998 KY26 was thought to be an asteroid until it was identified
as a dark comet in December.

"Jenny's definitely right about the entrapped hydrogen. Nobody had
thought of that before," he said. "Between discovering other dark comets
in the solar system and Jenny's awesome idea, I think it's got to be
correct. Water is the most abundant component of comets in the solar
system and likely in extrasolar systems, as well. And if you put a water
rich comet in the Oort cloud or eject it into the interstellar medium, you should get amorphous ice with pockets of H2." Because H2 should form in
any ice-rich body exposed to energetic radiation, the researchers suspect
that the same mechanism would be at work in sun-approaching comets from
the Oort cloud at the outer reaches of the solar system, where comets
are irradiated by cosmic rays, much like an interstellar comet would be.

Future observations of hydrogen outgassing from long-period comets could
be used to test the scenario of H2 formation and entrapment.

Many more interstellar and dark comets should be discovered by the Rubin Observatory Legacy Survey of Space and Time (LSST), allowing astronomers
to determine if hydrogen outgassing is common in comets. Seligman has calculated that the survey, which will be conducted at the Vera C. Rubin Observatory in Chile and is set to become operational in early 2025,
should detect between one and three interstellar comets like 'Oumuamua
every year, and likely many more that have a telltale coma, like Borisov.

Bergner was supported by a NASA Hubble Fellowship grant. Seligman
was supported by the National Science Foundation (AST-17152) and NASA (80NSSC19K0444, NNX17AL71A).

* RELATED_TOPICS
o Space_&_Time
# Asteroids,_Comets_and_Meteors # Sun # Astronomy
# Solar_Flare # Solar_System # Astrophysics #
Extrasolar_Planets # Northern_Lights
* RELATED_TERMS
o Comet o Comet_Shoemaker-Levy_9 o Comet_Hale-Bopp o
Interstellar_medium o Sun o Solar_flare o Jupiter o Outer_space

========================================================================== Story Source: Materials provided by
University_of_California_-_Berkeley. Original written by Robert
Sanders. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* An_artist's_depiction_of_the_interstellar_comet_`Oumuamua ========================================================================== Journal Reference:
1. Jennifer B. Bergner, Darryl Z. Seligman. Acceleration of
1I/`Oumuamua
from radiolytically produced H2 in H2O ice. Nature, 2023; 615
(7953): 610 DOI: 10.1038/s41586-022-05687-w ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/03/230322140338.htm

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